A representative self-locking knee joint of the prior art is shown in Patent Specification No. GB-B-2099708 and employs a sliding locking plunger pulled along its vertical line of action by a release cable, an upper member or knee ball being received in a housing forming part of the lower or shin member, and the knee pivot being between bearings to opposed sides of the knee housing. A similar joint in which a head of a lower memver fits between bifurcations of an upper member with a pin positioned at a posterior location is shown in U.S. Pat. Nos. 4,149,280 and 4,283,800 (Wilson), but there is no provision for locking the joint in extension.